(1) Field of the Invention
The present invention relates to a novel polyester block copolymer and an elastic yarn composed thereof. More particularly, the present invention relates to a polyester-polyester block copolymer having an excellent elastic recovery performance and a high light resistance and chlorine resistance, and an elastic yarn formed by using this block copolymer.
(2) Description of the Related Art
It is known that a polyester block copolymer is now used as a molding resin, as an elastomer characterized by a thermoplastic property, and this polyester elastomer consists of two kinds; i.e., a polyether ester block copolymer and a polyester ester block copolymer.
The polyether ester block copolymer comprises an aliphatic polyether such as polytetramethylene glycol and an aromatic polyester such as polybutylene terephthalate. This polymer is thermoplastic and has a high crystallization speed and a high heat resistance, and therefore, is widely used. Nevertheless, this polymer is defective in that its resistance to oxidative reaction is very low. For example, if this polymer is prepared and molded in the absence of a stabilizer, the surface becomes slippery and cracking occurs within less than several months. Namely, this polymer cannot be practically used without the aid of a stabilizer.
The addition of a stabilizer, however, gives rise to various problems; for example, if a molded body maintained at a high temperature is cooled, the stabilizer bleeds out to the surface and the molded body often appears to be fogged, which reduces its commercial value. Furthermore, if the molded body is used in the presence of NO.sub.x, a yellowing thereof sometimes occurs, and again, its commercial value is reduced. Moreover, it is obvious that, in medicinal application, the presence of a stabilizer that is easily extracted is not preferable.
A block copolymer comprising an aliphatic polyester and an aromatic polyester such as polybutylene terephthalate is known as the polyester ester block copolymer. The oxidation resistance of this polymer is higher than that of the polyether ester copolymer, but the polymer is defective in that the hydrolysis resistance is poor. Since there is no stabilizer able to improve the hydrolysis resistance, although a stabilizer able to improve the oxidation resistance is known, it has been considered difficult to improve the hydrolysis resistance. Further, although the oxidation resistance of the polymer is higher than that of the polyether ester copolymer, the oxidation resistance is still unsatisfactory, and thus a stabilizer must be used.
A spandex yarn has been used as an elastic yarn because it has an excellent elastic recovery performance, and an elastic yarn composed of a polyester block copolymer has been used recently only in restricted application fields. The spandex yarn, however, is defective in that, because of a poor wet heat resistance, a mixed fabric of the spandex yarn with a polyester fiber is difficult to dye, a yellowing thereof readily occurs under an irradiation with light, and the spandex yarn is unsatisfactorily used for swimming wear or the like because it has a poor chlorine resistance. Therefore, a practical use of the spandex yarn is very limited.
To overcome these defects, various investigations have been made into the preparing of an elastic yarn by using the above-mentioned polyester block copolymer, but only an elastic yarn composed of a polyether ester copolymer having an improved wet heat resistance has been put to practical use. This is because all of the obtained elastic yarns have an inferior spandex performance, especially a poor elastic recovery, but if this defect is overcome, the excellent properties of the polyester block copolymer will be come available and an elastic yarn free of the defects of a spandex yarn will be obtained.
The properties of the elastic yarn, such as light resistance and chlorine resistance, are substantially determined by the kind of soft segment, and therefore, a novel soft segment different from that of the spandex yarn is required.
The heretofore investigated polyester ester block copolymer comprising a polyester as the soft segment has not been put to practical use as a fiber, because the preparation of the polymer per se is difficult and the performance of the elastic yarn is still poor, and even if characteristics are manifested by the formation of the polymer, the defects are also clearly manifested and this hinders a practical use thereof. For example, the specification of U.S. Pat. No. 3,037,960 discloses a polyester block copolymer comprising a polyester having a melting point lower than 50.degree. C. and a polyester having a melting point higher than 200.degree. C., and represented by the formula of --(R'COOROCO) (where --R-- represents an aromatic diol residue). In this U.S. patent specification it is taught that the block copolymer is superior to the spandex yarn in color stability, light stability, and oxidative stability, and is superior to a polyether in oxidative stability. Although almost 30 years have passed since the issuance of this U.S. patent, the block copolymer has not been practically used. The reason for this is construed, from examples of the patent specification, to be that (1) the hydrolysis resistance is poor, (2) the tensile recovery is lower than that of the spandex yarn, and (3) the strength is low. Furthermore, the specification of U.S. Pat. No. 4,031,165 discloses a polyester block copolymer comprising an aliphatic polyester and an aromatic polyester, but it is considered that, when the polymer is used in the form of a fiber, a problem of a low hydrolysis resistance arises. The specification of U.S. Pat. No. 3,446,778 discloses a polyester block copolymer comprising a hexamethylene terephthalate/isophthalate/aliphatic dicarboxylic acid copolyester and polymethylene terephthalate, which resembles the block copolymer of the present invention, but this block copolymer also has not been put to practical use. According to the disclosed process, the two polymers are bonded through a chain extender, and it is considered that, since the chain extender is included in the polymer chain, the hydrolysis resistance and elastic recovery are lowered.
Another reason why a practical use thereof is inhibited is that the elastic recovery and the resistance to agglutination at the molding or spinning step are contradictory requirements. Namely, if an attempt is made to obtain a product having a good elastic recovery, especially in the form of an fiber, an agglutination occurs and the yarn cannot be released.
As pointed out hereinbefore, a polyether, for example, polytetramethylene glycol, often used as the soft component, has a very poor oxidation resistance and low light resistance and chlorine resistance and the like, but these defects can be overcome by use of a stabilizer. Nevertheless, the use of a stabilizer leads to problems such as a discoloration by nitrogen oxide (NO.sub.x) or a copper ion, and further, even if a stabilizer is used, a satisfactory light resistance or chlorine resistance cannot be obtained. In case of the spandex yarn, an aliphatic polyester is used as the soft component, for improving the chlorine resistance, but in this case, mildewing and hydrolysis occur. Therefore, if the problems of mildewing and hydrolysis can be solved, the application field of such a yarn will be broadened, and the durability thereof will be improved.
The conventional techniques and the problems thereof have been described above with regard to an elastic yarn as an example, but similar problems arise with a molded article and a film. The film, in particular, can be regarded as substantially equal to the fiber. In many cases, an elastic recovery performance is not required for a molded article, but since a stabilizer is used, problems such as a bleed-out of the stabilizer arise. Therefore, the development of an elastomer that can be used without the use of a stabilizer is desired.